PhD Oral Exam, Neil Dasgupta, Tuesday, April 19th, 2011, 9:00 am, bldg 660-220
dasgupta at stanford.edu
Mon Apr 18 12:18:43 PDT 2011
Stanford University Ph.D. Oral Examination
Title: “Quantum Confinement Structures for Efficient Energy Conversion”
Neil P. Dasgupta
Department of Mechanical Engineering
Advisor: Prof. Fritz B. Prinz
Date: Tuesday, April 19th, 2011
Time: 9:00 am (Breakfast/refreshments at 8:45pm)
Location: Mechanical Engineering Research Lab (MERL, bldg. 660), Conference room (203)
Quantum confinement (QC) structures present several opportunities for developing next-generation energy conversion devices, due to the ability to tune the electronic and optical properties of a material as a function of size and shape. In the case of solar cells, the ability to tune the bandgap and modify the kinetics of charge relaxation in QC structures suggests the ability to improve broadband solar absorption, and potentially develop devices in excess of the traditional ~32% efficiency limitation of single bandgap devices. However, the ability to fabricate QC solar cells presents several challenges due to the necessity to precisely control feature size and separation, as well as the inherent challenge to extract excited charge carriers from these features before significant recombination losses occur.
In this study, atomic layer deposition (ALD) was studied as an enabling technology for fabricating 3-D nanostructured QC solar cell architectures. In the first part, the ability to fabricate PbS quantum wells by ALD was developed, and QC effects on the bandgap were demonstrated as a function of film thickness. Additionally, a new technique to directly deposit quantum dots (QDs) was developed by utilizing nucleation-limited growth during the initial ALD cycles. The evolution of the size and shape of these dots was studied using plane view transmission electron microscopy (TEM). Dome shaped QDs which were formed by ALD with subsequent annealing were studied using the STEM-EELS technique, allowing for a measurement of nanoscale bandgap variations within an individual QD.
In the second part, 3-D QC solar cell architectures were developed by ALD. ALD of Al-doped ZnO (AZO) was studied as a transparent electrode material, and p-Si/n-ZnO diodes were fabricated by ALD to aid in charge extraction from QC layers. A fully integrated QD solar cell with PbS QDs integrated into a p-Si/n-ZnO diode with an AZO top electrode was demonstrated. Finally, the fabrication of 3-D nanostructured templates, including conducting nanowires and etched quartz substrates was studied in order to aid in light scattering and minimize the required thickness of the QD layer, thereby minimizing the diffusion length required for charge extraction. To demonstrate the power of the ALD technique, a single layer of PbS QDs was uniformly deposited on the surface of Si nanowires, and photoluminescence measurements were performed to demonstrate the ability to modify their optical properties through QC effects.
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